Stratigraphic architecture of the world’s oldest shale gas play: The 1400-1200 Ma Velkerri and Kyalla formations in the Beetaloo Sub-Basin

The Mesoproterozoic Velkerri and Kyalla formations in the Beetaloo Sub-basin in northern Australia contain the world’s oldest shale plays. In unconventional exploration, the main challenge is the identification of sweet spots from which hydrocarbons can be produced economically. In fine-grained siliciclastic intervals, the distribution of these sweet spots is mainly controlled by the evolution of the sedimentary system and its effects on organic matter distribution. The aim of the present work is therefore to reconstruct the stratigraphic architecture of the Velkerri-Kyalla interval and to integrate its stratigraphic evolution with chemostratigraphic and chronostratigraphic frameworks. Based on core descriptions and well correlations, we reconstructed the facies distribution within the Velkerri-Kyalla interval along two regional well sections. This allowed for the recognition of stratigraphic stacking patterns and to identify multiple depositional sequences. Three stratigraphic orders are observed within the Velkerri-Kyalla formations. The studied interval is interpreted to be deposited during one second-order depositional sequence, containing four nested third-order depositional sequences. These, in turn, are composed of thirteen nested fourth-order transgressive-regressive sequences. By integrating the sequence stratigraphic architecture with available geochronological constraints, we postulate a chronostratigraphic framework for these Mesoproterozoic strata. The Velkerri Formation was likely deposited between 1400 Ma and 1280 Ma, and the overlying Kyalla Formation between 1250 Ma and 1190 Ma. Furthermore, by coupling the sequence stratigraphy, chemostratigraphy and existing provenance studies, and chronostratigraphy, we were able to construct regional paleogeographic maps to illustrate the evolution of the basin in the broader context of the Mesoproterozoic of northern Australia.